I feel like the problem is simply... it should just be a file. And if it were a file, you could sync it with dropbox or OneDrive. And if you can do that, they can't make money selling their own cloud service for their local-first app. More specifically, if you can edit different parts of a same document on different devices, then the document should be split across multiple files that can be synced independently, e.g…
How do these services resolve conflicts?